The paper presents a collection of 93 different bugs, detected in formal verification of 65 student designs that include: 1) single-issue pipelined DLX processors; 2) extensions with exceptions and branch prediction; and 3) dual-issue superscalar implementations. The processors were described in a high-level HDL, and were formally verified with an automatic tool flow. The bugs are analyzed and classified, and can be used in research on micro-processor testing.